# Skill: Database Migration

## Description
Handles safe database schema changes and data migrations.

## When to Use
- Adding/modifying database tables or columns
- Changing relationships between entities
- Data transformations

## Instructions

### Migration Rules
1. **Always reversible** — every migration needs an `up` and `down`
2. **One change per migration** — don't bundle unrelated schema changes
3. **Test on dev first** — never run untested migrations in production
4. **Backup before production** — always have a rollback plan
5. **No data loss** — add columns as nullable first, then backfill

### Drizzle Workflow
```bash
# Generate migration from schema changes
npx drizzle-kit generate

# Review generated SQL in drizzle/ folder
# Apply migration
npx drizzle-kit push

# Or use migrate for production
npx drizzle-kit migrate
```

### Naming Convention
`YYYYMMDD_HHMMSS_description.sql`
Example: `20260212_143000_add_user_roles.sql`

### Dangerous Operations Checklist
- [ ] Column rename → Create new, copy data, drop old (3-step)
- [ ] Column type change → Verify data compatibility first
- [ ] Drop table → Confirm no foreign key references
- [ ] Add NOT NULL → Set default or backfill first